At the end of a round I had a squad of two basic infantry troops and a mortar unit which I set to OP Fire. The two infantry units ended up getting killed leaving my mortar crew in OP Fire. According to the rules mortars can't OP Fire.

In this case do I fatigue the mortar team or do I remove the OP Fire token from that unit and attack with my mortar squad?

 I would say fatigue the unit... 

I agree, and have done this in games where this has come up. Normally the only way to remove a OP fire token is via a card from one of the Strategy decks. Losing your troops should be a disadvantage not an adavantage.

I would say that it willl stay in Op Fire (without an opportunity of resolving) till the end of the turn or till the moment of using an appropriate card, that removes op Fire token. So it's not fatigued in the meaning of spotting another mortar crew for instance 

San said:

I agree.

We always interpreted the rule to mean that the squad containing the mortar couldn't even be put into opfire.

TheKaiser33 said:

Well, the rules say:

No Opportunity Fire: A mortar figure cannot participate in opportunity fire (even if other figures in its squad are doing so).

That clearly limits the restriction to the mortar figures. If the squad also contains other figures, they may use Op Fire.